[dart] Generated code does not compile in flutter. Ends with simple_type_transformer exception

Generated code for dart does not run in flutter. Getting this error message which I don't know what i means. Using dart-2 preview.

compiler message: file:///home/xx/Software/flutter/.pub-cache/hosted/pub.dartlang.org/dartson-0.2.7/lib/src/simple_type_transformer.dart:8:20: Error: The parameter 'value' of the method '_SimpleTypeTransformer::encode' has type dartson::_SimpleTypeTransformer::T, which does not match the corresponding type in the overridden method (dynamic).
compiler message: Change to a supertype of dynamic (or, for a covariant parameter, a subtype).
compiler message:   dynamic encode(T value) => value;
compiler message:                    ^
compiler message: file:///home/xx/Software/flutter/.pub-cache/hosted/pub.dartlang.org/dartson-0.2.7/lib/type_transformer.dart:9:11: Context: This is the overriden method ('encode').
compiler message:   dynamic encode(T value);
compiler message:           ^
Compiler failed on /home/xx/workspace/smb-flutter/lib/main.dart

FAILURE: Build failed with an exception.

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel beta, v0.5.1, on Linux, locale en_US.UTF-8)
[✓] Android toolchain - develop for Android devices (Android SDK 27.0.3)
[✓] Android Studio (version 3.1)
[!] VS Code (version 1.25.0)
[✓] Connected devices (1 available)

! Doctor found issues in 1 category.

Reproduce the Flutter build failure using the provided environment details, then inspect dartson's lib/src/simple_type_transformer.dart and lib/type_transformer.dart at the reported method signatures. Compare the overridden encode declarations and determine what must change for the generated Dart code to compile successfully in Flutter.